This invention relates to visual observations in space and more specifically relates to a space deployable and assemblable starshade.
A starshade is a large space-based light shield which serves as an external occulting mask in an external stellar coronagraph system. Its function is to cast the shadow of a star on a telescope. Its optical element properties are derived primarily from the starlight diffraction cancelling shape around its perimeter, although it must also minimize other brightness noise sources such as stray sunlight. Contrary to typical optical elements such as reflecting optics whose optical properties are derived though very finely figured mirrored surface finish, which strongly effects optical wavefront error, the starshade surface has no optical wavefront impact. No light reflected directly off of the starshade's roughly planar surface is directed into the telescope nominally, meaning the roughly planar surface can be fairly rough (un-flat) and the external stellar coronagraph system's imaging capabilities are not affected.
The starshade and a corresponding space-based telescope are separated by a significant distance and together form a two-spacecraft observation system. The starshade is positioned precisely between the space-based telescope and a nearby star to block the direct light from the star before it reaches the telescope. The field of view of the telescope is centered on a nearby star and set just wide enough to observe exoplanets (or other objects) orbiting the star, while minimizing the view of bright objects not orbiting the star (such as background galaxies). The nearby star's shadow on the telescope (provided by the starshade) facilitates detection of the exoplanets/objects with much lower brightness than the central nearby star (in visible wavelength band light). This technique allows astronomers to directly observe and spectrally characterize the light reflected off of the exoplanets/objects, which can be as small as Earth sized and in the nearby star's habitable zone (the distance from the star required to have liquid water temperatures on the exoplanet's surface). Starlight reflected off the exoplanets/objects in the direction of the telescope is permitted to pass outside of and very close to the starshade without being shaded. The starshade system is also designed and used in a manner required to suppress sunlight which is either reflected off of or transmitted through the starshade and into the telescope. Sunlight suppression must be at least as efficient as the suppression of the nearby star, in the center of the field of view of the telescope; where a typical starlight suppression requirement of 1010 (or one part in ten billion permitted to enter the telescope) is required for the direct imaging technique described to be successful.
Flower-shaped petals of the starshade enhance the ability of the starshade to more effectively block the starlight by reducing the constructive bending of light waves from the star at the edges of the starshade, into the telescope. This enhances a darker shadow cast towards the telescope by the starshade. While a simple circular shaped starshade would be much simpler to construct, its shading capabilities would be many orders of magnitude worse than the flower shaped petals, due to diffraction at the hypothetically circular starshade edge, permitting constructive bending at the circular edge and re-imaging of the starlight within the telescope; peaking with a bright spot at the center (referred to as the Spot of Arago).
The “Space-Based Occulter” in U.S. Pat. No. 8,167,247, assigned to the assignee of the present invention, describes such a starshade. It is carried into space using a single launch, as a collapsed umbrella-like configuration in which a covering is attached to a folded mechanical structure so that to achieve the final deployed state in space the folded mechanical structure is unfolded into a final position causing the connected covering to expand accordingly. While the resulting deployed occulter satisfies the starlight blocking goal, it will be appreciated that going from the stored state during its passage into space to its final deployed state requires significant movement and interaction of its mechanical structure and interconnected petals and coverings.